Brain anatomy and sensorimotor gating were compared in 21 adults with Asperger’s syndrome and 24 controls, all of normal IQ, aged 18-49 years. Brain anatomy was studied using quantitative MRI, and sensorimotor ‘gating,’ or ability to inhibit repetitive thoughts, speech and actions, by measuring prepulse inhibition of startle (PPI). In PPI the startle response to a strong stimulus is muted or inhibited when momentarily preceded by a weak stimulus (the prepulse). The PPI was tested in a subset of 12 patients with Asperger’s syndrome and 14 controls. Control but not Asperger subjects had significant age-related reductions in volume of cerebral hemispheres and caudate nuclei. Asperger subjects had significantly less grey matter in fronto-striatal and cerebellar regions, and widespread differences in white matter, compared to controls. Sensorimotor gating was significantly impaired in Asperger’s syndrome. The difficulties inhibiting repetitive thoughts, speech and actions, characteristic of subjects with Asperger’s syndrome, are associated with anatomical abnormalities in fronto-striatal pathways that result in defective sensorimotor gating. 
COMMENT. Prepulse inhibition of startle response (PPI) is dependent in part on intact fronto-parietal pathways (Bubser and Koch, 1994 and others). The above study demonstrates that subjects with Asperger’s syndrome have a significant impairment in sensorimotor gating, or an inability to inhibit repetition of thoughts, speech and actions. This characteristic behavior of Asperger and other autistic patients is associated with reductions in volumes of frontostriatal and cerebellar grey matter and diffuse white matter changes.